To produce the steel with phosphorus less than 0.007% (mass percent),the double slag process has been optimized in basic oxygen furnace (BOF).Dephosphorization and decarburization are performed in sequence in the same converter.Deslagging is conducted twice,each at the end of dephosphorization and decarburization,by tilting the BOF.Production was tested in a 180 t BOF.Mass percent of phosphorus in steel at BOF blow end has been made to as low as 0.004 2% after optimizing operational parameters.Factors affecting dephosphorization performance were studied.Before intermediate deslagging,dephosphorization is determined by the mass transfer of phosphorus from metal/slag interface to bulk slag phase rather than equilibrium thermodynamic conditions.After intermediate deslagging,dephosphorization is determined by the equilibrium thermodynamic conditions such as slag composition and temperature. To produce the steel with phosphorus less than 0.007% (mass percent), the double slag process has been optimized in basic oxygen furnace (BOF). Dephosphorization and decarburization are performed in sequence in the same converter. end of dephosphorization and decarburization, by tilting the BOF. Production was tested in a 180 t BOF. Mass percent of phosphorus in steel at BOF blow end has been made to as low as 0.004 2% after optimizing operational parameters. Factors affecting dephosphorization performance were studied.Before intermediate deslagging, dephosphorization is determined by the equilibrium transfer of phosphorus from metal / slag interface to bulk slag phase rather than equilibrium thermodynamic conditions .After intermediate deslagging, dephosphorization is determined by the equilibrium thermodynamic conditions such as slag composition and temperature.